There is a growing marketplace for argon gases and liquids with little to no radioactive content from Argon-39 (39Ar). Argon gas is generally collected from the atmosphere. However, atmospheric argon contains a small amount of 39Ar and is, therefore, radioactive. This radioactivity which is typically found in atmospheric argon is in low amount but interferes with uses of argon, such as, for example, in dark matter detectors, and in proportional counters used to search for 37Ar (a marker for nuclear test ban violations).
Argon gas that is collected from sources deep underground has been found to not contain 39Ar and, therefore, is not radioactive. Specifically, 39Ar in the atmosphere is derived from the interaction of 40Ar with cosmic rays. As such, 39Ar is much less in underground sources that are protected from cosmic rays. However, in producing underground argon, one must take care that there is not an infiltration of too much (or any) atmospheric argon. The infiltration could be from a longer term geologic phenomenon, or associated with the production process. Thus, it would be useful to have a simple, rapid test to exclude the possibility of atmospheric Argon during production. However, direct testing for 39Ar is a lengthy process, and thereby does not lend itself to a production-level test.
The difficulty of measuring Ar39 is difficult to overstate. In the atmosphere the Ar39 to Ar40 ratio is 10^-15. The only way to measure the Ar39 directly is to build a multi million dollar ultra sensitive radiation detector. Applicant knows of only 2 published efforts to measure Ar39 in underground argon. Method one measured the scintillation light from 1 kilogram of liquefied argon, and measured it for a month or more, and then did not directly succeed. The conclusion was that the researchers did not see it. The other measurement used 157 kg of the underground Ar39 (the world's supply). This was an international collaborative effort involving on the order of 50 scientists.
The current inventor recognized the need for a more rapid test than is available for 39Ar content, and that the ratio of 36Ar to 40Ar provides a useful surrogate test. In the atmosphere, the portion of Argon that is 36Ar is substantially higher than in underground Argon. Similarly, the ratio of 38Ar to 40Ar provides another useful surrogate test.